1. Field of the Invention
The present invention relates to an apparatus for forming a thin film from a semiconductor such as hydrogenated amorphous silicon, or an insulator such as hydrogenated amorphous carbon, or the like on a substrate, and in particular, relates to a thin film formation apparatus in which a chemical vapor deposition (CVD) process is utilized for the formation of a thin film.
2. Description of the Related Art
Various types of CVD processes are known for the formation of thin films on substrates, for example, the thermal decomposition CVD process, the photo-assisted CVD process, and the plasma-assisted CVD process.
In the thermal decomposition CVD process, a gas of a starting material (hereinafter referred to as a starting gas) is thermally decomposed and activated within a reactor vessel to produce active species as decomposition products, and these active species are deposited on a substrate to form a thin film thereon. In general, in this type of process, the substrate is heated to a high temperature so that the decomposition of the starting gas occurs in the vicinity of a surface of the substrate. For this reason, the substrate must be formed of a material able to endure heating to a high temperature. Substrate materials usable in the thermal decomposition CVD process are restricted to materials which cannot be fused or deformed by heating. Also, if the substrate is overheated during the thermal decomposition CVD process, the thin film formed on the substrate may be unsatisfactory because a component of the deposited compound may be released therefrom due to the overheating of the substrate.
In the photo-assisted CVD process, a starting gas is decomposed and activated by light energy within a reactor vessel to produce active species as decomposition products, and the produced active species are deposited on a substrate as a thin film. The starting gas is irradiated by light having a wavelength in the ultraviolet band, which is liable to be absorbed by the starting gas to decompose the latter so that the active species are produced from the starting gas. In general, the source for the irradiation light may be a low-pressure mercury-vapor lamp, a deuterium discharge tube, or an ArF excimer laser device. The low-pressure mercury-vapor lamp and the deuterium discharge tube emit light continuously, but the intensity thereof is small and thus it is difficult to efficiently carry out the decomposition of the starting gas, resulting in a poor growth rate for the thin film. On the other hand, it is impossible to obtain a continuous light from the ArF excimer laser, since the ArF excimer laser beam is a pulsed light having a pulse frequency of less than 1 kHz. The life of any active species thus produced is shorter than one cycle of the pulse frequency, resulting in a deterioration of the characteristics of the deposited thin film. In addition, during photo-assisted CVD processes, the light is introduced into the reactor vessel through a glass window so that the decomposition of the starting gas is liable to occur in the vicinity of the glass window, whereby the produced active species may be deposited on the glass window, and thus, as more of the active species is deposited on the glass window, the introduction of the light through the glass window is impeded.
In the plasma-assisted CVD process, a starting gas is introduced into the plasma zone generated within a reactor vessel by utilizing either a direct current (DC) discharge, a radio frequency (RF) discharge or a microwave discharge. The introduced starting gas is caused to collide with electrons and ions in the plasma zone so that the starting gas is dissociated to produce active species which deposit on the substrate in the form of a thin film.
When a DC discharge is utilized in the plasma-associated CVD process, the formation of excellent thin film conductors can be carried out because a stable DC discharge may be easily maintained during the formation of thin film conductors, but it is impossible to carry out the formation of excellent thin film semiconductors or insulators because the DC discharge cannot be stably maintained during the formation of semiconductors and insulators. This is caused by the fact that the active species for the formation of semiconductors or insulators deposit not only on the substrate but also on the electrodes, so that electric resistance is gradually increased between the electrodes. Accordingly, the DC discharge plasma-assisted CVD process cannot be utilized for the formation of thin film semiconductors or insulators.
An RF discharge can be utilized in the plasma-assisted CVD process to form both thin film conductors and thin film semiconductors or insulators. Industrially, the RF discharge plasma-assisted CVD process is most widely used because the starting voltage at which the RF discharge occurs is relatively low, and because a stable plasma can be easily obtained over a wide area. Nevertheless, since the density of the plasma generated by RF discharge is low (10.sup.8 to 10.sup.10 cm.sup.-3), the growth rate of the thin film is low and the starting gas cannot be effectively consumed.
The RF discharge plasma-assisted CVD process is frequently used to form amorphous thin film semiconductors from materials such as hydrogenated amorphous silicon (a--Si:H), and in general, the formation of such amorphous thin film semiconductors is carried out in such a manner that the pressure within the reactor vessel (the pressure of the starting gas) is maintained within a range of from 0.1 to 10 torr.
When the formation of the amorphous thin film semiconductor is carried out within a high pressure range of from about 0.3 to 10 torr, the growth rate of the thin film is relatively high, but the amorphous thin film semiconductor may be defective. In particular, when the high pressure range of from 0.3 to 10 torr is employed, the active species dissociated from the starting gas collide with each other and the starting gas molecules so that high-molecular weight compounds are produced as a fine powder within the reactor vessel. For example, when the formation of a hydrogenated amorphous silicon (a--Si:H) thin film is carried out within the high pressure range of from 0.3 to 10 torr, high-molecular weight powders of Si.sub.n H.sub.m (n, m=natural numbers) deriving from a starting gas such as SiH.sub.4, Si.sub.2 H.sub.6 or the like are produced, and these high-molecular powders may deposit together with the active species on the substrate, resulting in defects in the hydrogenated amorphous silicon thin film. Since the high-molecular powders also may deposit on the discharge electrodes and on the inner wall surfaces of the reactor vessel, the reactor vessel must be cleaned after every formation process, to remove the high-molecular powders. Furthermore, when the CVD reactor vessel is in communication with another reactor vessel through the intermediary of a valve, so that the substrate can be moved from one reactor vessel to another reactor vessel, the high-molecular powder may deposit on the valve, and thus the valve becomes unable to operate normally.
On the other hand, when the formation of the amorphous thin film semiconductor is carried out within a low pressure range of from about 0.1 to 0.3 torr, it is possible to prevent the production of the high-molecular weight powder because the probability of collisions among the active species and the starting gas molecules is considerably lowered due to the low pressure of the starting gas, whereby the formation of an excellent amorphous thin film semiconductor can be carried out without defects. This, however, means that the growth rate of the thin film is very low, and that the starting gas cannot be effectively consumed.
A microwave discharge can also be utilized in the plasma-assisted CVD process to form either thin film conductors or thin film semiconductors and insulators. In general, the microwave discharge plasma-assisted CVD process is performed by introducing microwaves into a reactor vessel through a waveguide. Although an antenna might be used for generating microwaves within a reactor vessel, the discharge plasma can be obtained only at a localized zone in the vicinity of the antenna, so a waveguide is generally used. The microwave discharge can be stably maintained even under a very low pressure of 10.sup.-4 torr by utilizing a magnetic field (electron cyclotron resonance (ECR)), so that the probability of collision between the starting gas molecules and electrons is considerably enhanced, whereby not only can the rate of the thin film deposition be increased, but also the starting gas can be effectively consumed. In the microwave discharge plasma-assisted CVD process, however, it is impossible to obtain a wide plasma zone because the cross-sectional area of the plasma zone is restricted to that of the waveguide due to the strong directivity of the microwave. By incorporating the cavity resonator into the reactor vessel, the plasma zone can be widened to the inner space of the cavity resonator, but the size of the cavity resonator is restricted by the frequency of the microwave used. For this reason, the microwave discharge plasma-assisted CVD process cannot be utilized to form a thin film on a large scale substrate. In addition, in general, the characteristics of thin films formed by the RF discharge plasma-assisted CVD process are inferior to those of thin films obtained by the microwave discharge plasma-assisted CVD process.